In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In a twin-wire draw, the groups of drying cylinders include two wires which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws. The free draws are susceptible of fluttering which may cause web breaks, in particular since the web is still relatively moist and, therefore, has a relatively low strength. For this reason, in recent years, increasing use has been made of a single-wire draw in which each group of drying cylinders has only one drying wire. The web runs on support of the drying wire through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, and whereas, on the reversing cylinders or rolls arranged between the drying cylinders, the web remains at the side of the outside curve. Thus, in a single-wire draw, the drying cylinders are placed outside the wire loop and the reversing cylinders or rolls are situated inside the drying wire loop.
In prior art normal groups having a single-wire draw, the heated drying cylinders are typically arranged in the upper row and the reversing cylinders are therefore arranged in the lower row. The upper row and lower row are generally horizontal and parallel to one another. The assignee's Finnish Patent No. 54,627 (corresponding to U.S. Pat. No. 4,202,113, the specification of which is hereby incorporated by reference herein) describes an arrangement wherein normal groups having a single-wire draw and so-called inverted groups having a single-wire draw are arranged one after the other. In the inverted groups, the heated drying cylinders are arranged in the lower row and the reversing suction cylinders or rolls are arranged in the upper row. This arrangement enables a principle objective to be achieved, i.e., to dry the web symmetrically from both of its sides.
With respect to additional prior art, reference is made to published International Patent Applications WO 88106204 and WO 88106205 (assigned to Beloit Corp.) which describe dryer sections having normal and inverted cylinder groups.
Accordingly, in the following descriptions, the terms "normal (drying) group" and "inverted (drying) group" are used to denote the cylinder groups having a single-wire draw as described above, as such is accepted terminology to those skilled in the art.
In dryer sections that comprise inverted and normal drying groups, various problems have occurred. The present invention is directed toward a resolution of these problems. For example, problems have been encountered in the runnability of the dryer section and in the threading of the web, problems arising from differences in the speeds of different wires, problems in the removal of broke especially in inverted groups, as well as problems related to the control of transverse shrinkage of the web. These problems tend to become worse as the running speed of the paper machine becomes higher.
With respect to prior art involved in and related to the present invention, reference is made to the following patent publications and articles published in journals:
W. Haessner, "Trocknungstechnik und deren Entwicklung"; Das Papier 44, 10A, 1990; PA1 "The Valmet Sym-Run Concept", Paper Asia, May/Jun 1992; PA1 J. Yli-Kauppita, "Dryer Section for High Speed Paper Machines", Proceedings of the Helsinki Symposium of Alternate Methods of Pulp and Paper Drying, Helsinki June 4-7, 1991; PA1 Sam Palazzolo, "No-draw drying", Tappi Journal, September 1990; PA1 W. Leitenberger, "Die Contirun-Trockenpartie fur schnellen, sicheren Bahnlauf", Das Papier, Heft 6, 1992; PA1 U.S. Pat. Nos. 3,753,298, 3,868,780, 4,602,439, 4,972,608, 4,982,513, 5,022,163, 5,065,529, 5,146,696, and 5,177,880; PA1 V. Korhonen and A. Kuhasalo, "Ropeless tail threading from press to reel", World Pulp & Paper Technology 1993; PA1 H. Lepisto und P. Eskelinen, "Verbesserung der Lauffahighkeit schneller Papiermaschinen mit Hilfe neuer Ventilationseinrichtungen", Das Papier 1985, Heft 10A; PA1 Lindberg, Juppi, Eskelinen, "High Speed Dryer Section Developments for Sheet Stability", 78th Annual Meeting, Technical Section CPPA, 1992.
With respect to the prior art closely related to the invention, reference is further made to the assignee's Finnish Patent Application No. 906216 (corresponding to U.S. patent application Ser. No. 07/808,161, the specification of which is hereby incorporated by reference herein), in which a method is described for drying a web in the dryer section of a paper machine, in particular for reducing the tendency of curling of the paper web. In the method described in FI '216, the paper web is dried on drying cylinders, against whose heated faces the paper web is pressed by means of a drying wire. In the dryer section, groups of drying cylinders are used, in which twin-wire draw and/or single-wire draw is/are applied. In this method, it has been considered novel that, in the dryer section, hot water steam is fed substantially onto the entire width of the paper web. By means of this steam, tensions that have been formed or that tend to be formed in the fiber mesh in the paper web are relaxed by means of heat and moisture in the area of their formation or substantially immediately thereafter.
Further, in FI 906216, a dryer section of a paper machine is described, intended for carrying out the above method, and which comprises one or preferably several successive drying groups which consist of drying cylinders and wire guide rolls and/or reversing cylinders. In the dryer section, a single-wire draw and/or a twin-wire draw is/are applied. In this dryer section, it has been considered novel that at least one, and preferably several, steam supply box is arranged in the dryer section and extends substantially across the entire transverse width of the paper web to be steam-treated. The steam box communicates with a steam source, and the steam box comprises a counter-face which, together with the free face of the paper web that runs at its proximity, forms a contact-free steam-treatment gap.
Further, it is known in the prior art, in a dryer section, to use devices for regulating the transverse moisture profile, such as infrared and/or moistening devices. In the prior art, by means of these devices, attempts have not been made to control the moisture profile of the paper in the z-direction, i.e., in the direction of thickness, but they are used exclusively for the control of the transverse moisture profile of the web. Further, in SC paper machines ("SC" stands for SuperCalendered), a procedure is known from the prior art in which the paper web is dried to an excessive dryness in order to obtain a sufficiently good moisture profile, whereupon the paper web is re-moistened to a moisture content optimal in view of the calendaring process. Thus, the function of these moistening devices is merely to increase the ultimate moisture content of the paper, and not to equalize its moisture profile in the z-direction.
In the prior art, a dryer section is known which is exclusively composed of the above drying groups with a single-wire draw. In these groups, between the contact-drying cylinders placed in the upper rows in the groups, normal small diameter suction rolls that are provided with inside (internal) suction boxes have been used. One particular prior art dryer section of interest in a dryer section supplied by, e.g., J. M. Voith GmbH, and situated at PM 1, Stora Feidmuehle, Reisholz, Duesseldorf, Germany.
A drawback of these small diameter suction rolls is the high requirement of negative pressure and suction energy, because, owing to the small diameter of these rolls, high centrifugal forces arise on these rolls which tend to separate the web from the drying wire. By means of the curve sectors of small radius, the suction rolls also produce a rather large relative difference in speed between the drying wire and the web, which is in many respects unfavorable. Further drawbacks include the wear of the seals at the suction box inside the suction rolls and the repeated requirement of servicing of these seals as well as the high noise level. This prior art dryer section has also required development in the respect that, since in all the groups with single-wire draw, the drying effect is applied on the contact-drying cylinders to only one side of the web, i.e., to the lower side of the web, and therefore the web tends to be dried asymmetrically in the z-direction. To an even greater extent, the web is dried at the web side placed in contact with the faces of the contact-drying cylinders. Thus, one object of the present invention is to suggest novel solutions for these problems.
In the following description, problems and requirements of the further advanced development of the prior art dryer sections, e.g., such as those attended to by the patents and papers mentioned above, will be discussed in greater detail. As background information, it should be stated that the highest web speeds of paper machines are currently already about 25 m/s (meters per second). However, in the near future, the range of web speeds will be from about 25 m/s to about 40 m/s in future paper machines. In this case, the dryer section of a paper machine will constitute a bottle-neck for the runnability of a paper machine to an increasing extent. Moreover, with the use of the prior art dryer concepts, the dryer section will become quite long.
In the inverted drying groups mentioned above, in the case of web breaks, a problem arises in the removal of broke since inverted groups are not self-cleaning by the force of gravity.
The above problems and some other problems are emphasized further if, in the dryer groups with a single-wire draw, small-diameter suction rolls proper are used that are provided with an inside suction box. In order to eliminate this problem, in some machines, it has been even necessary to open some group gaps and to lower the level of negative pressure in the suction rolls.